Structural Deterioration of the Produce: The Breakdown of Cell Wall Components - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 178

detectable from the second day itself and continues throughout the course of softening.
Treatment with 1-methylcyclopropene (1-MCP) inhibits both ripening/softening as well as
MiExpA1 transcript and protein accumulation. It is suggested that MiErpA1 expression is
ethylene dependent, and its expression increases with the progression of ripening (Sane et al.,
2005).

8.11 Transgenic approaches to enhance cell wall integrity

The functions of several enzymes active in ripening fruit have been explored using transgenic
strategies. Table 8.3 summarizes phenotype of some of transgenic plants underexpressing
these enzymes and proteins. Antisense inhibition of PG had slight effect on fruit softening
but significantly changed depolymerization of pectins resulting in increase in juice viscosity
(Thakur et al., 1997). Transgenic plants expressing an antisense gene of PME showed over
95% reduction in fruit PME activity and resulted in marked improvement in juice viscosity
and increased total soluble solids in fruits (Tieman et al., 1992; Gaffe et al., 1994, Thakur et
al., 1996a, b). Fruit softening was not affected during normal ripening but showed breakdown
of tissue integrity after extended storage (Tieman and Handa, 1994).
Homology-dependent silencing of TBG 1 resulted in about 90% reduction in its transcript
accumulation but had affected neither the total exogalactanase activity, cell wall galactose
content, or fruit softening (Carey et al., 2001). These results suggest that TBG1 either
does not contribute significantly to total -galactosidase activity or has activity that is

specific to a minor cell wall component. Suppression of TBG3 by its antisense RNA gene
resulted in several changes including up to 75% reduction in extractable exogalactanase
activity, simultaneous reduction in TBG 1 and TBG 4 transcript levels, and increased cell wall
galactose content. These change had little effect on ripening-related fruit softening, but fruit
deteriorated slowly during long-term storage (de Silva and Verhoeyen 1998). Juice from
these transgenic fruits showed increase in insoluble solids and viscosity. Transgenic tomato
fruits expressing about 1,500 bp of TGB4 in antisense orientation showed strong reduction
in TGB4 transcripts and about 90% reduction in extractable exogalactanase activity (Smith
et al., 2002). Compared to wild type free galactose levels in all TBG4 antisense lines
were lower at mature green stage 4, but in ripening fruits. Also, the total fruit cell wall
galactosyl contents were not affected by the antisense gene. All of the antisense lines
had reduced free galactose levels at mature green stage 4, but levels comparable with
control during ripening. Total cell wall galactosyl contents in the antisense fruit were not
significantly different from control fruit. Fruits from several independent transgenic were
firmer than control. Fruits from the transgenic line, designated 1-1, exhibiting maximum
reduction in TBG4 transcripts and exogalactanase activity, and the highest galactosyl residue
content during early stage of ripening were 40% firmer than control (Smith et al., 2002).
By expressing TBG4 in yeast, Smith and Gross (2000) have confirmed that it encodes
both a galactosidase and an exogalactanase. Taken together above results provide a strong
evidence for the involvement of

β

β -galactosidase in cell wall modification leading to fruit

softening.
Results with other genes have indicated that specific members of the

-galactosidase,
expansin, and PL gene families partially regulate softening in tomatoes or strawberry (Brum-
mell et al., 1999; Jimenez-Bermudez et al., 2002; Smith et al., 2002). However, functional
analyses of specific PGs, PMEs, XTHs, and EGases in tomatoes, strawberries, and peppers

β